Security has been stepped up in state capitals and key installations on Independence Day eve to thwart any terror strike, with an elaborate ground-to-air vigil being maintained in Delhi where the Prime Minister will unfurl the tricolour at the Red Fort on Wednesday.

With insurgents outfits and Maoists calling for a boycott of celebrations, tens of thousands of police and paramilitary forces have been deployed and patrolling intensified in the North-eastern states and those hit by Left wing violence, including Chhattisgarh.

In Jammu and Kashmir, security measures have been intensified across the Valley following stepped up militant attacks over the past week, with police and other security forces resuming random frisking of pedestrians and searching of vehicles.

A thick security blanket has been thrown across Mumbai with over 40,000 policemen deployed on surveillance duty against the backdrop of Saturday's violence during a rally at Azad Maidan.

A 'red alert' has already been sounded across violence hit Assam, which witnessed a few IED blasts targeting security personnel during the past couple of weeks, particularly in Goalpara district.

Thousands of Delhi Police and paramilitary personnel were deployed in and around Delhi, especially near the Red Fort and sharpshooters of the NSG deployed on the high-rises near the 17th century Mughal monument, officials said.

According to the officials, the localities around Red Fort have been sanitised and informers have been deployed to keep a check on any movement of anti-national elements.

Specialised units like Quick Reaction Teams, SWAT and Vajra are being deployed.

An intense vigil will also be maintained on key installations like Parliament complex, international airport, railway stations, inter-state bus terminals and Metro stations.

Besides patrolling of the skies by helicopters, an air defence mechanism will also be put in place around the venue to thwart any terror attacks.

In Chhattisgarh, with Naxals calling for a boycott, security vigil has been tightened, especially in border districts, Additional Director General of Police Ram Niwas said.

In Manipur, the coordination committee (CORCOM) of seven major insurgent outfits including United National Liberation (UNLF), Revolutionary People's Front (RPF), People's Revolutionary Party of Kangleipak (PREPAK) and Kanglei Yawol Kann Lup (KYKL) has called for a boycott. Security forces have intensifies patrol in Imphal and police and security posts in have also been instructed to step up checking.

The anti-talk faction of ULFA in Assam has also called a customary state-wide bandh on the day.

"The upper Assam belt is under special scanner as our intelligence feedback suggest serious attempt by the ULFA to create disturbances there. We are ready to deal with the situation and foil any such attempt," an official said.

Railway Police are on high alert in Karnataka in the wake of threats by suspected terror groups operating from Mysore to carry out blasts on some trains plying from here on the eve of Independence day, a top Railway official said.